The present disclosure relates to a variable gain amplifier used for a wireless communication system, or the like, capable of amplifying a received signal with a desired gain.
In recent years, mobile phones offer not only a verbal communication function, but also wireless communication functions such as digital television, FM radio, Bluetooth (registered trademark), wireless LAN, and GPS. In order to meet the requirements of the miniaturization and price reduction of wireless communication devices and modules included therein, high-performance circuit systems which have not been integrated, are now integrated on a single wireless communication chip. Variable gain amplifiers for amplifying a high-frequency signal received with the antenna are one kind of such high-performance circuit systems.
In a wireless communication device, a variable gain amplifier amplifies a received signal inputted from the antenna without adding noise to the signal, and transmits the signal to the circuits that follow the variable gain amplifier. A variable gain amplifier is included in most wireless communication devices to achieve a reliable wireless communication.
When there is a longer distance between a wireless communication device and a base station, the intensity of the high-frequency signal received with the antenna is very low. In this case, the variable gain amplifier increases the gain. By contrast, when the distance therebetween is shorter, the intensity of the received signal is too high for the wireless communication device. In this case, the variable gain amplifier sets the gain to a value lower than 0 dB so that the output of the receiver does not become saturated. In order to demodulate a received signal, the variable gain amplifier needs best possible low-noise characteristics and linearity.
The Japanese terrestrial digital broadcasting service for mobile devices (ISDB-T, the so-called “1-SEG”), which has been spread rapidly, is characterized as its broadband transmission. An IC chip for receiving 1-SEG is often built in a mobile phone. Therefore, even with a high-frequency filter between the antenna and the 1-SEG tuner, various interfering waves within and outside the bandwidth are inputted into the tuner via the antenna and due to the radiation within the terminal. Furthermore, the gain of the variable gain amplifier has to be changed dynamically in order to improve communication quality. In recent years, both front-end elements (analog circuits) including a variable gain amplifier and back-end elements (digital circuits) including an OFDM demodulation unit are integrated into a single IC chip to reduce the mounting area and manufacturing cost of IC chips for receiving 1-SEG. Therefore, a high-performance variable gain amplifier having a smaller mounting area has to be manufactured at low costs by using CMOS manufacturing process, which has large process variations.
In conventional variable gain amplifiers, a plurality of cascode stage transistors connected in parallel with a power source and a load resistor are provided. The cascode stage transistors are selectively turned on and off to increase and decrease the gain while the amount of current passing through amplification transistors is constant (for example, see Japanese Patent Publication No. 2007-259297).